1. Field of the Invention
The present invention relates to a diversity receiving device in spread spectrum communications.
2. Description of the Related Art
For a receiving circuit used in a mobile communications device such as a cellular phone, etc., a diversity receiving device adopting a plurality of antennas is frequently used in order to prevent a reception quality from being degraded due to a changing transmission path. The diversity receiving device is used to lower the probability that the quality of a reception signal is degraded when receiving a faded wave. In spread spectrum communications, antenna diversity for selecting the outputs of a plurality of antennas, and path diversity for distinguishing between reception signals according to a time difference thereof, are implemented. With the antenna diversity, wireless receiving units are respectively arranged for a plurality of antennas, and all of receiving systems are continually operated, so that signals of good reception quality are selected and despread.
For the path diversity, its effects are obtained by separating a reception signal with the use of an arrival time difference between electronic waves, which is caused by different transmission paths, and by despreading and combining the separated signals.
FIG. 1 is a block diagram exemplifying the configuration of a conventional CDMA diversity receiving device.
In FIG. 1, signals received by two antennas A and B are detected by wireless receiving units 83-1 and 83-2. Within the wireless receiving units 83-1 and 83-2, the signals received by the antennas A and B are first amplified by variable amplifiers 82-1 and 82-2 that are controlled by ALC controllers 80-1 and 80-2. Mixers 81-1 and 81-2 convert the RF band signals into baseband signals by multiplying the signals from the variable amplifiers 82-1 and 82-2 by the cyclic waves which correspond to the signal carrier waves and are output from a local oscillator not shown. The baseband signals are detected by orthogonal-signal-detector 84-1 and 84-2. The reason why the wave detectors are represented as the orthogonal-signal-detectors 84-1 and 84-2 is that both of CDMA signals I and Q received by the antennas A and B are assumed to be detected. Their actual operations are similar to those of a normal detector. They are specifically called orthogonal-signal-detectors, since the CDMA signals I and Q are orthogonal each other. However, if a reception signal is not composed of the signals I and Q, the orthogonal-signal-detectors 84-1 and 84-2 shown in this figure can be replaced with normal detectors. The ALC controllers 80-1 and 80-2 perform a control so as to make the outputs from the wireless receiving units 83-1 and 83-2 constant by using the outputs of the orthogonal-signal-detectors 84-1 and 84-2 as inputs, and by providing a driving signal to the variable amplifiers 82-1 an 82-2.
The reason that the ALC controllers 80-1 and 80-2 are arranged as described above is to improve conversion accuracy when an analog signal is converted into a digital signal by an A/D converter which is not shown in this figure and is arranged at the stage succeeding the wireless receiving units 83-1 and 83-2. Namely, the intensities of the signals received by the antennas A and B vary due to an influence of fading, etc. In A/D conversion, the intensity of a signal to be converted is estimated, and the number of bits of a digital signal after being converted is determined. However, if the intensity of a reception signal is much lower than an estimated value, also the intensities of the signals output from the wireless receiving units 83-1 and 83-2 become very much low. As a result, a minimum unit used when the intensity of an analog signal is digitized becomes unsuitable, so that an analog signal with a low intensity cannot be digitized with high accuracy. Namely, when an analog signal with a low intensity slightly changes, it is represented as a digital signal with no change. This is because the minimum unit of digitization of an analog value is larger than the amount of this change, which is relatively large for a signal with a low intensity and includes information. That is, digitization noise becomes relatively large for an analog signal with a low intensity, so that a meaningful change of a signal value is not regenerated as a digital signal.
Accordingly, the wireless receiving units 83-1 and 83-2 detect all of reception signals after amplifying them to signals of a same level under the control of the ALC controller 80-1 and 80-2, and transmits the detected signals to a circuit at a succeeding stage. After the detected signals output from the wireless receiving units 83-1 and 83-2 are converted into digital signals by the A/D converter not shown, they are input to a branch selecting unit 85. The branch selecting unit 85 selects either of the signals from the antennas A and B, and transmits the selected signal to a matched filter 86. The branch selecting unit 85 selects either of branches A and B (the system which receives the signals from the antenna A is referred to as a branch A, while the system which receives the signals from the antenna B is referred to as a branch B) with a branch selection instructing signal input from a searcher 87. The signal output from the branch selecting unit 85 is despread by the matched filter 86 with a despreading code instructed by the searcher 87, and correlation values at respective timings are calculated. The correlation values obtained by the matched filter 86 are transmitted to the searcher 87. The searcher 87 determines whether the currently obtained correlation values are either the values from the branch A or those from the branch B based on the contents of the branch selection instructing signal transmitted to the branch selecting unit 85. If the searcher 87 determines that the currently obtained correlation values are those from the branch A, the correlation values are stored in an antenna A delay profile memory 88-1. If the searcher 87 determines that the correlation values are those from the branch B, the correlation values are stored in an antenna B delay profile memory 88-2.
Then, the searcher 87 respectively reads the correlation values from the antenna A delay profile memory 88-1 and the antenna B delay profile memory 882, and obtains correlation values which are equal to or larger than a predetermined value and their despreading timings. The despreading timings can be easily obtained if the correlation values are stored in time series and the timing of the first correlation value is stored as memory contents. A plurality of the despreading timings thus obtained by the searcher 87 are timing lags delayed by multipath. And, since the matched filter 86 despreads signals with the same despreading code, they are the signals of the same channel. The searcher 87 transmits the despreading code and the despreading timings obtained as described above to a finger allocating unit 90.
The finger allocating unit 90 allocates the despreading code and the despreading timings, which are transmitted from the searcher 87, to a plurality of fingers 91-1 through 91-n, and makes the plurality of fingers 91-1 through 91-n perform a despreading process respectively. Each of the fingers 91-1 through 91-n is composed of a sliding correlator and a synchronous-signal-detector, and is intended to despread the received signal with the despreading code allocated by the finger allocating unit 90 at respectively allocated despreading timings, and to detect a transmitted signal. Namely, in a CDMA communication made by a portable terminal, despreading timings may differ due to multipath fading even if signals are of the same channel. Therefore, the searcher 87 selects a plurality of despreading timing candidates from among the correlation values obtained from the matched filter 86, and makes the fingers 91-1 through 91-n perform a despreading process at respective timings to detect signals. As shown in this figure, the signals despread by the respective fingers 91-1 through 91-n are signals which are directly input from the wireless receiving units 83-1 and 83-2 to the finger allocating unit 90 via the A/D converter not shown. As a result of despreading these signals with the same despreading code, signals of the same channel are regenerated. The despread signals are input to an ALC cancelling unit 94. The ALC cancelling unit 94 obtains the information indicating which branch signal is amplified by what multiple, and cancels an amplification result obtained by the ALC control, for example, by multiplying the signal amplified by a multiple xe2x80x9cnxe2x80x9d by 1/n. The reason why such cancelling is made is that noise accompanying a signal with a low intensity is amplified more than that accompanying a signal with a high intensity when the signal with the low intensity is amplified by the ALC control to the same level as that of the signal with the high intensity, and despread signals are much influenced by the noise if they are combined unchanged. For this reason, the ALC cancelling unit 94 cancels the signal amplification made by the ALC control, and inputs the signals to a combining unit 92. The combining unit 92 adds the synchronously detected signals from the fingers 91-1 through 91-n, whose amplification operations are cancelled by the ALC cancelling unit 94. Since the respective fingers 91-1 through 91-n perform a despreading process at different timings when the synchronously detected signals are added, the timings at which the synchronously detected signals are output may differ. Accordingly, the combining unit 92 comprises a RAM, etc., in which the synchronously detected signals from the respective fingers 91-1 through 91-n are once stored. All of the signals are output from the RAM in synchronization with one another, and added. With the addition operation, the degradations of the signals transmitted over a plurality of paths can be averaged. The added signals are then decoded by a decoder 93, and the decoded signals are transmitted to a data receiving unit (not shown) at a succeeding stage. The above described method for arranging a plurality of fingers 91-1 through 91-n and for almost simultaneously performing a despreading process and synchronous-signal-detection at different timings is called RAKE reception.
One of vital objectives in the development of a spread spectrum communications portable terminal is to save space and power.
Therefore, the conventional diversity receiving circuit requiring wireless receiving units which are difficult to be put into LSIs, and the number of which is equal to the number of antennas, is a big obstacle to this objective.
An object of the present invention is to provide a diversity receiving circuit which can save space and power.
A diversity receiving device according to the present invention, which uses a plurality of antennas in spread spectrum communications, comprises: a plurality of antennas; a switching unit for switching between the signals from the plurality of antennas, and outputting the switched signals; a wireless receiving unit for receiving the signal in which the signals from the plurality of antennas are alternately arranged in time series, and for detecting the signal; a separating unit for separating the detected signal including the signals from the plurality of antennas into the respective signals from the plurality of antennas; an extracting unit for extracting despreading timings from the signals output from the separating unit; and a decoding unit for despreading the detected signal including the signals from the plurality of antennas at the despreading timings extracted by the extracting unit, for RAKE-receiving the despread signal, and for decoding the signal.
A diversity receiving method according to the present invention, which uses a plurality of antennas in spread spectrum communications, comprising the steps of: (a) switching between the signals from the plurality of antennas, and outputting the switched signals; (b) receiving the signal obtained in the above described step (a), in which the signals from the plurality of antennas are alternately arranged in time series, and detecting the signal; (c) separating the detected signal, which is obtained in the step (b) and includes the signals from the plurality of channels, into the respective signals from the plurality of antennas; (d) extracting the despreading timings from the signals obtained in the step (c); and (e) despreading the detection signal of the signals from the plurality of antennas at the despreading timings extracted in the step (d), RAKE-receiving the despread signal, and decoding the RAKE-received signal.
According to the present invention, even if a plurality of antennas are arranged for diversity reception, only one configuration for detecting the signals from the plurality of antennas can be arranged and shared for the antennas. Even if the number of antennas increases, the configuration required for making detection remains to be only one. As a result, the hardware configuration can be simplified, which leads to a reduction in device size. Furthermore, since only one configuration for making detection is arranged for a plurality of antennas, thereby consuming less power.